Ministers have been ordered to 'come clean' over when they first knew about the Volkswagen diesel emissions scandal after a string of fresh developments today.

Labour said the government's response had been 'unacceptable' after it emerged the department for transport had been sent evidence that diesel cars were being fiddled to pass tests almost a year ago – but failed to do anything about it.

The scandal erupted in the US after diesel emissions research by the International Council on Clean Transportation found a huge discrepancy between official tests and real-life emissions on the road.

Patrick McLoughlin's department was sent evidence that diesel cars were being fiddled to pass emissions tests almost a year ago - but failed to do anything about it

But 11 months ago his department was informed about 'strong evidence of a real-world NOX [nitrogen oxides] compliance issue' in the EU and US.

It tested 15 vehicles and found they produced an average of seven times the legal limit for nitrogen oxides, which the government says cause 23,500 premature deaths a year in the UK, the Times reported today.

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Labour's shadow transport secretary Lilian Greenwood today said it was 'unacceptable' that the government had waited so long to take action.

She said: 'Ministers must come clean and admit when they were first told about the diesel emissions scandal.

'The International Council on Clean Transportation, the body which helped to expose the problem, warned a year ago that dangerously high levels of nitrogen oxide emissions were not confined to America.

Volkswaggon has admitted installing software into its diesel cars sold in the USA that manipulated emissions test results

She accused the government of performing 'a series of screeching U-turns' on the issue.

'After initially saying that only the European Commission could conduct an investigation, Patrick McLoughlin has been forced to backtrack, but after this latest revelation there can be little faith in the department's response.'

A transport department spokesman said the UK Government has been 'at the forefront of action at a European level' to introduce updated emissions testing.

He added that the ICCT report published in October last year 'did not identify the vehicles tested'.

Volkswagen chief executive Martin Winterkorn has resigned following the scandal

VW has admitted that 11 million vehicles worldwide were fitted with sophisticated software which conned testers in the US into believing their vehicles met environmental standards.

The Environmental Protection Agency said 482,000 of the German car-maker's 2009-15 models in the US were fitted with the defeat device to switch engines to a cleaner mode when they are undergoing official emissions testing.

Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.

VW has not revealed whether cars in the UK are affected but Germany's transport minister, Alexander Dobrindt, said 2.8 million vehicles in his country have the software.

He added that VW vans as well as cars are caught up in the scandal.

Mr McLoughlin said on Thursday that the Government was taking the ''unacceptable actions of VW extremely seriously''.

He announced that the UK regulator would investigate and re-run laboratory tests and called on the EU to conduct a Europe-wide investigation to find out if cars have been fitted with defeat devices on the continent.

The British Lung Foundation urged the Government to commit to 'routine, independent real-world testing on all cars'.

Volkswagen chief executive Martin Winterkorn resigned over the emissions scandal but insisted he was not aware of 'any wrongdoing on my part''.

According to reports the 68-year-old could receive a severance package of up to two years of his annual salary, which would be worth over £22 million.